4 Tube Amp From Scratch

[J M Fahey]

the tubes take what they need from the voltage

You are quoting Karl Marx´s law, applied to Electronics.
We run by Ohm´s law here.
Sorry , couldn´t resist
Sorry 2: why oh why did I have to use the word resist in this context. ?

Sorry.
Now to serious matters.
1) won´t waste space answering your last post, let´s concentrate on filaments.
2) *If* you get a *real* transformer (most emphatically not an autoformer of any kind) which has an 185V secondary (dunno, it´s what you mentioned) .... or you mean 185V DC ? In which case your transformer would need around 125/130V AC PLUS
3) a 50V at 150mA secondary strictly for the 50*5 output tube filament PLUS
4) a 6.3V 600mA secondary for the 6V filament tubes, then you could power everything without problems.
If you switch the 50*5 tube to a more normal 6V one, you won´t need the 50V filament.
Although the 6V secondary should supply around 1A, the thing becomes much easier.
You can keep your output transformer, chassis, etc.
Good luck.

[R.G.]

Upon reflection, you need to read this:
The All American Five Radio.

[Chuck H]

Well, i figured since the autoformer takes 110-220v that a 1:1 would be cool, and that way i could use the same heater setup as is already installed on the radio (and could use it next time i'm in the States with it, too, as opposed to having a 2:1).

The rub with the 1:1 transformer is that where your at now it will supply 220V to the filaments which require 110V. You NEED a 2:1 or it will fail.

[TubeNoob]

You are quoting Karl Marx´s law, applied to Electronics.
We run by Ohm´s law here.
Sorry , couldn´t resist
Sorry 2: why oh why did I have to use the word resist in this context. ?

Classic!!!! You've almost left me incapacitated (har har!)....But like i said...IM STUBBORN! (and prefer original stuff), tho nonetheless, i need the lessons....

Ok, so yes i have a 'transformer' (read: not an autoformer) which puts out those secondary voltages. Is there a way to work it?

RG - thanks, i read that article and was familiar with most of it, if there was a specific part of it you think i missed, i would super appreciate it if you'd point it out - again, just trying to learn.

Ok, so guys, tell me why i can't just smack that 12ax7 in with the 50b5 and 35w4 since they're all 150ma tubes (and the 50 and 35v tubes are clearly outside of a 10% voltage tolerance yet work together on the string in this original radio) and then i just shoot 6v (separately) over to the 6at6?

I am really just asking to learn, not be a prick! :-)

[diagrammatiks]

someone else will have to verify for me...

but it seems that the 3 preamp tubes... are all 6.3v heated.

so that transformer is running through the rectifier tube and then the power tube in series to bring the voltage down and then having the rest in parallel?

[R.G.]

RG - thanks, i read that article and was familiar with most of it, if there was a specific part of it you think i missed, i would super appreciate it if you'd point it out - again, just trying to learn.

Good. I was just wanting you to have the background. That set of tubes was selected to work together at least partially because they had heater voltages that added up to nearly 110Vac and they all had the same (150ma, was it?) currents. This let them tie the entire heater string across the AC line and not need a heater transformer winding, and therefore be cheap. The article also hits on the safety issues, which are severe.

Ok, so guys, tell me why i can't just smack that 12ax7 in with the 50b5 and 35w4 since they're all 150ma tubes

You can. As long as they are all 150ma (heater current) tubes, they will be happy running on the same current in a series string. Notice that in a series string they *literally* have the same current flowing in the heaters. And when the rated 150ma is flowing, the heater voltage will be the specified value. So if you put the heaters of a 50B5 and a 35W4 in series, they work right when there is 85Vac across the heaters. Add a 12AX7, and the series heater voltage is then 97Vac. Add two 6Vac rated heaters and you get to 109Vac. In series, the heater voltages add.

(and the 50 and 35v tubes are clearly outside of a 10% voltage tolerance yet work together on the string in this original radio) and then i just shoot 6v (separately) over to the 6at6?

This is because they're hooked in series, not parallel. The 50B5 needs a heater voltage of 50Vac, +/- 10%. The 35W4 needs a heater voltage of 35Vac +/-10%. You can't hook those in parallel and have both tubes be happy, because 50Vac is not within 10% of 35Vac either way. You can hook them in series because they both want the same current (150ma). Each tube needs the voltage across its heater to be rated voltage +/-10% to get the right current. The tubes are hooked in series, and have the same current.

Your comments about heater voltages and currents bother me. They make me think you're missing the difference between things hooked in parallel and things hooked in series. Things in parallel have the same voltage across them and may have different currents in each one. Things in series have the same current through them but may have a different voltage across each one.

Maybe it will help for you to think of the heaters as resistors. A 50Vac heater that pulls 150ma is a "resistance" of 50/0.15 = 333 ohms. A 35Vac heater that pulls 150ma at 35V is a "resistance" of 35/0.15 = 233 ohms. A 12Vac heater that pulls 150ma is a "resistance" of 12/0.15 = 80 ohms and for 6V, 40 ohms.

You can take the transformer you have, and hook up the secondaries so they add to any convenient voltage which provides the sum of the heater voltages you hook in series. If you have 50+35+12+6 = 103V needed for heaters, connect up the next larger voltage you can get, then cram in a resistor to drop the excess voltage at 150ma, and you're done. If you need 103Vac for heaters and can only connect up secondaries to get 112Vac, then you need to eat up the 112-103V =9Vac excess; this would be done by a resistor of 9Vac/0.15A = 60 ohms.

[J M Fahey]

If you have 50+35+12+6 = 103V needed for heaters, connect up the next larger voltage you can get, then cram in a resistor to drop the excess voltage at 150ma, and you're done.

True.
Only tubenoob stubbornly insists on using his 6AT6 (because he has one).
But it´s a 300mA tube, not a 150mA one, so it can´t be series fed but will need its own 6.3V winding.
And now the others do not add up to close to 110V AC anymore. Fail.

What worries me a lot is that until now it has never been posted a full schematic of what he´s trying to build, only bits and pieces.
Even more because it leaves away the most dangerous parts:
> Power line connection
> Power transformer.
> Filament connection.
> Grounding.
> Fusing.

And he´s showing both "stubbornness", ignorance, and despise for advice given, a very dangerous combination.

I think that doing for him the homework he refuses to learn how to do properly, and telling him : "here, do this ... and this ... and this ... " puts the good intentioned fellow who tries to help in a dangerous position.

We are working with tubes here, not a 9V battery powered pedal.
Slight difference.

jm2c

[TubeNoob]

Thanks to all of you for your posts and education!

If you have 50+35+12+6 = 103V needed for heaters, connect up the next larger voltage you can get, then cram in a resistor to drop the excess voltage at 150ma, and you're done

R.G., I'm a little confused on this since the 6v tube needs 300ma and according to what i understood you guys were saying, it can't be done in series with 150ma tubes.


Ok, so to clarify a few things - i wasn't thinking of running them in parallel, nor of just using the autoformer, and...the original radio has a mix of different current tubes (150ma and 300ma together) with an extra line coming from the autoformer between the two different current tube groupings.

I am trying to understand that 2nd line from the autoformer - i would assume its adding more current for those 300ma tubes.

And - i'm wondering if i can do this:

220v 1:1 Transformer >> Autoformer >> 150ma tubes >> 2nd line from autoformer >> 300ma tube >> Light or resistor.

Please check out the attached graphic for a visual representation of this.

Basically, i'm asking if the autoformer (powered by an isolation transformer) can be left 'as is', or if adding that 12ax7 before the 2nd autoformer line (and removing the two 6v tubes after that line) is a prob.

If this isn't doable, i will partially concede defeat (!) and see what other output tube i have that's 6v, per JM Fahey's suggestion. My main interest boils down to the 6at6 and the tremolo.

[TubeNoob]

Whoa easy killer! I'm trying to understand this stuff, not get someone else to do my homework! Ignorance? Not ignoring anything, just trying to take it all in and find solutions. "despise" for advice given? Hardly! I have been most appreciative and thanked everyone at every turn and modified my schematic based on advice given.

And in my last post, i just said that i'm looking at going with YOUR suggestion of ditching the 50b5 for a 6v output tube if that's the only way to work it, so i'd like to think that disproves your theories about me at least a little!

Stubborn - yeah, i can be, so i admit it. Could also call it 'persistent' maybe, but hey.

I worked out a schematic for the amp, and now im focusing on the heater supply, maybe i should have done that first. As for power line, grounding and fusing - of course they will be included, but until i figure out if i can even use these tubes, its a little premature to spec out the power supply, is it not? Like i said, maybe that's backwards, i've heard you should build your amp based on your power supply, but again - i'm learning! Isn't that what the forums are all about?

And Bah Humbug to you too!

[J M Fahey]

Dear tubenoob
For one:
going by your last schematic posted, the 9th so far (count' em) , it still has errors.
Not small or typing errors but some which show you don´t understand Ohm´s Law, or what a transformer does.
Please don´t consider this an insult or a demeaning attitude at all, we all try to help here, but we are running against the brickwall of your not understanding or not following Electrical Laws.
Or not caring about them.
In the end it all boils to the same.
Consider your (incomplete) transformer diagram:
To begin with, some definitions:
transformers are voltage sources and have constant voltage between their taps.
So: your transformer has , between the two wires shown leaving it (you forgot to draw others which are very important, at least the one that´s grounded), 85V (AC) and it can supply, at least, 150Ma.
How do I know?
Because (originally) it´s feeding the 35W4 and the 50B5 filaments, which add up to 85V.
Your Mod suggests wiring those two wires across 3 tubes. You added a 12AX7.
Now you need 97V.
Your tubes will be severely undervoltaged (remember the 4th power emission Law).
I´m sure the 6AT6 (6V filament) , which is replacing three 6V tubes (18V in total) will *try* to dissipate 9X the correct heating (why 9X and not 3 X ? .... read some Electricity Book) and will blow in a spectacular flash of light.
We are on the 9th iteration and still with a ticking time bomb schematic !!!!!!!!!!!!!!!!!!!!!!!!

Yea, I know, you will smile, thank politely (that´s one in your favor ) and modify the schematic acordingly.
Fine.
Still new gremlins will rise their heads over and over, because of the previously mentioned reasons.
Now please, answer this one faithfully: if you started with a (non working) schematic and have modified it 10 times on our input ..... who is really doing this homework?

Not to mention that it still remains unfinished.
Have a Nice Christmas.

[R.G.]

R.G., I'm a little confused on this since the 6v tube needs 300ma and according to what i understood you guys were saying, it can't be done in series with 150ma tubes.

I wasn't speaking of your specific 6V tube, but some hypothetical 6V tube which uses only the 150ma in the series string.

If you have to use a 6V, 300ma heater tube, then you are correct, it can't be done simply by putting it in series with a 150ma string of heaters, because it won't have enough current to get hot enough.

If you have to use a 300ma heater (actually, of any voltage rating) you have to figure out a way to get it 300ma.

Ok, so to clarify a few things - i wasn't thinking of running them in parallel, nor of just using the autoformer, and...the original radio has a mix of different current tubes (150ma and 300ma together) with an extra line coming from the autoformer between the two different current tube groupings.

I am trying to understand that 2nd line from the autoformer - i would assume its adding more current for those 300ma tubes.

And to clarify, I was not looking at how to use the autotransformer, I was trying to get the general principles across. It is possible that they used the auto transformer to get extra current to some tubes. I'll go look.

And - i'm wondering if i can do this:
220v 1:1 Transformer >> Autoformer >> 150ma tubes >> 2nd line from autoformer >> 300ma tube >> Light or resistor.
Please check out the attached graphic for a visual representation of this.

Basically, i'm asking if the autoformer (powered by an isolation transformer) can be left 'as is', or if adding that 12ax7 before the 2nd autoformer line (and removing the two 6v tubes after that line) is a prob.

The answer is a solid "maybe". To calculate this, you have to really, really know the specifics of that autoformer. You need to know at least (1) the open circuit voltage across the whole thing that it was supposed to get, (2) the open circuit/no load voltage at each tap and (3) the resistance between each tap. This lets you do the calculations to know what the autoformer does under load, and will eventually let you calculate whether the autoformer could be used, perhaps with some outboard resistors to add current or subtract voltage from heaters outside the autoformer.

This calculation is not hugely complicated, but it does require some design experience and more technical detail than you have given us yet.

All of us have an interest in making sure you don't (a) burn up the toys when you first turn them on and more importantly (b) don't die in the resulting fire or get electrocuted when you turn it on. That's why you're hearing a lot of reticence and comments about things burning up. Your dedication to working it out is very good; but please understand that we want you to live, and that comes out in various comments.

[TubeNoob]

Hey JMFahey - Well of course i'll smile and thank you, i appreciate constructive criticism!

As i said in my first post, i'm throwing myself in the deep end as a means to learn, and received some great input and the suggestion to go ahead and build it from this forum. So i kept on truckin'. It is true i am not highly familiar with electronics or its laws, but again, i'm trying to learn; that's the only reason i'm doing this - it is certainly not that i don't care.

I count 6 versions of the main schematic and am now working on the power side, but anyway, how many posts and questions equal 'doing homework' on this forum? I thought i was just learning and improving my understanding, and given that people were still willing to help, (i believe you're the first to say ditch it), i figured i had some support to keep at it. If it just grates on your nerves too much, i'll completely understand if you don't visit this thread anymore. But truth be told, i've learned a helluva lot so far, including from you, and i appreciate the input and the opportunity to learn.

I did build a Princeton from scratch and mod'd it (Master vol, NFB on/off, Selectable tone-caps, Standby switch) so I've proven to myself that i can follow someone else's schematic - and learn only so much about tube electronics in the process compared to what this thread has taught me.

True, i did not include the other lines from the transformer and autoformer on my drawing, but i didn't think it was necessary in terms of working out the heater arrangements - as we all know those lines are there and must be used. They were only left out for convenience, not lack of knowing about them. I will be sure to include them in the future so there's no confusion.

So, regarding your last post - one thing i don't get is you're saying the voltages won't add up to be the same, but i was suggesting removing two 6v tubes and adding a 12v tube, which adds up to the same total voltage does it not? What am i missing here? Is this something to do with that 2nd line from the autoformer?

Anyway, as stubborn as i am, i did say that if there's just no way to work it, i can use other tubes, and dug thru my box to find a 6v6 and a 6x4 rectifier.

As i mentioned, i'm really interested in the 6at6 input stage (as in the Harvard and the Matchless Clubman) and the Kalamazoo tremolo circuit.

Merry xmas to u too, sir! And as always, thanks for your advice...!


RG - just saw your post, will study it shortly - thank you!

[jmaf]

Thanks JMaf and by the way, good luck on your open-source amp - super cool of you and that thing sounds amazing!

Thank you, glad you liked it. Let me just say you are being helped by some of the best minds in this business. Years ago I would send RG Keen a couple emails a year asking basic questions that I'd come across reading Geofex, he'd answer the emails with the same patience and technical sharpness he does here in public, even for my silly questions.

I can't stick around today, or I'd love to try and help more. But like I said you have the most senior folks in here helping you out, I'm sure it'll come out great. Merry Christmas everyone!

[TubeNoob]

Jmaf - thanks a lot for your post, i certainly appreciate that fact and it's given me the impetus to keep at it. I've been reading Geofex since i first started building circuits (albeit solidstate) and it's a huge honor to have RG as well as the rest of you throwing me your input.

I also greatly appreciate the concern for a) toys not blowing up b) me not blowing up!!! I do want to assuage concerns by saying that i, in fact, enjoy being alive, and also that i do not just build high voltage stuff and turn it on - when i built my Princeton i had an E.E. with me to make sure i didn't catch fire and to check things on the scope. Here in Italy I have another E.E. here to help with the dangerous stuff, and still, I am on the forums to get the idea to a point where its at least a viable schematic. And if guys like you give me the thumbs up, I am willing to commit the time and solder - but not before then. I don't need an amp so bad as to forget the safety factors.

RG - just spoke with my bud the E.E. and he will help me test out the autoformer per your suggestion. I also discussed with him the autoformer issue in general, and i do see now how there could be some adjustments necessary to make it work, if in fact it might work at all. The autoformer is bumping up the current for those 6v tubes, so if i remove 2 of them, i'd need resistance (as you pointed out) to take their place. Then to add the 12ax7 before that 2nd autoformer line, i'll need to know what kind of juice that part of the circuit is able to provide (again, in accordance with what you pointed out).

And last but not least, JM Fahey - thanks for being that 'gremlin' on my shoulder to keep things in check ;-)

Happy Holiday to you all.

[mooreamps]

Only that I'm not a big fan of this type of volume/tone stack arrangement... In this circuit, when you turn the volume control (R5) to max, it will short out C4 on the tone stack, and then there goes your tone stack.. I'd move the volume control over on the other side V2a.

-g